Deformation-based micro surface texturing system

ABSTRACT

A compact deformation-based micro-texturing apparatus and method employ flexure bearing houses for rotatably supporting opposite ends of each of a first (e.g. upper) roll and a second (e.g. lower) roll to provide a working roll gap between the rolls, wherein at least one of the rolls has one or more micro surface features to plastically deform a surface of a workpiece deformed by rolling action in the roll gap. An electrical current may be passed through the workpiece to assist micro deformation. A roll gap adjusting device is operably associated with the first and second flexure bearing houses for adjusting the roll gap dimension to the final depth of the micro surface features to be imparted to the surface of the workpiece by the rolling action.

This application claims benefits and priority of provisional applicationSer. No. 61/459,668 filed Dec. 16, 2010, the entire disclosure of whichis incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a deformation-based micro-surfacetexturing apparatus and method useful for efficiently and economicallyfabricating surface textures, such as micro channels and micro dimples,on the surface of thin metallic or non-metallic sheet materials.

BACKGROUND OF THE INVENTION

Creating micro-scale surface textures on the exterior of interface partshas potentially promising energy and biomedical applications whereincreation of new energy resource and saving of energy consumption can beenabled through innovative micro-forming technologies. Themicro-manufacturing technology which leads to reduction of energyconsumption and environmental pollutions will bridge macro-scaleproduction and nano-scale enabling science to make it possible forrealizing innovative ideas on energy sustainability. For example,arrayed microchannel device (AMD) is a stack of laminas contactedthrough channel heads. Each lamina consists of hundreds of microchannelsthat facilitate heat exchange or mass transfer. AMD has a direct impacton increasing energy efficiency in the application areas, such asdistributed power generation, hydrogen generation and fuel cells.Micro-channel heat exchangers have demonstrated heat fluxes 3 to 5 timeshigher than conventional heat exchangers [reference 1]. As energyefficiency has become increasingly important due to escalating fuelcosts, the desire to reduce friction and wear of contacting surfaces hasintensified. It is estimated that up to one third of all energy usageworldwide is used to overcome friction, resulting in a potentiallysignificant energy savings with even moderate friction reduction[reference 2]. Surface texturing is a nontraditional technique forfriction reduction; rather than maintaining a very smooth surface,dimples are intentionally created on the surface of a part in slidingcontact, resulting in a significant friction reduction [references 3-7].These dimples serve as micro-reservoirs for the lubricant, resulting ina reduction in lubricant leakage. During sliding, lubricant pressurebuilds up in the dimples, which in turn helps to create a separationbetween the contact parts. The dimples also function as receptacles fordebris and wear particles, eliminating potential scratching of thesubstrate surface during the relative motion of the interface parts.

Economically sustainable manufacturing methods are needed to enable thecommercialization of these applications. Established fabrication methodsfor micro channels and micro dimples include laser ablation, micromachining, and photochemical machining. Laser ablation utilizeshigh-intensity laser pulses to incrementally ablate minute segments ofthe substrate material to create the desired feature geometry [reference3]. Laser ablation can easily texture nearly any material. However, itrequires expensive equipment and surface finish of product is generallypoor if a fast process is desired. Micro machining removes material froma substrate using cutting or milling to leave a desired geometry. Microincremental forming [reference 8], one type of die-less micro forming,forms micro channels of various profiles by depressing and drawing thesingle point tool along the thin metal sheet. These three methods allowfor accurate shapes and tolerances, but are fairly time consuming andexpensive. They are only suitable for fabricate prototypes.

Currently, mass production of micro channels and dimples are enabled byphotochemical machining, which places an etching mask on the top ofspecimen to selectively etch the material via etching chemicals[reference 9]. Photochemical machining is able to create almost anygeometry that mimics a pre-made pattern. However, the fabrication costof this process is still too high when high-volume of shims is needed.Besides, photochemical machining also poses material waste andenvironmental concerns. Most of the chemicals such as cleaningsolutions, etchants, strippers etc. are hazardous liquids. Therefore,handling and disposal of them are very costly. The ideal manufacturingmust be able to minimize the impact to the environment. Therefore, amethod which is capable of efficiently and economically fabricatingmicro channels and micro dimples is needed.

If the deformation-based micro surface texturing process successfullymeets the tolerance requirement, it will lead to many launches ofapplications in the areas of energy generation, energy utilization andoptical illusions. However, during the forming process, the bump orpile-up is built up around the dimples. Futamura et al. [reference 10]developed a micro dimple forming process to improve the anti-seizingproperties of sliding surfaces of mechanical components and reducefrictional resistance. They found bumps building up when forming thedimples on A2017 aluminum alloy and S45C carbon steel pipes. Afterburnishing the bumps, the depth of the dimples on A2017 pile almost keptthe same, while the dimples on S45C pipe disappeared totally. The reasonfor this difference is that to remove the bump of carbon steel which hasa much higher Young's modules and strength than those of the aluminumalloy, burnishing roll has to go deeper, which flattened not only thebumps but also the dimples. Cao et al. [reference 11] investigated theeffect of relative velocity on shape distribution of the micro dimplesas the relative velocity between the forming tool and sample affectsfrictional force. They found a clear relationship between relativevelocity and frictional force, and its significant effect of relativevelocity on the final profile of the dimple.

An economically sustainable micro-manufacturing method embodyingdeformation-based micro texturing would be beneficial and would enablethe commercialization of the above-described applications.

SUMMARY OF THE INVENTION

The present invention provides a deformation-based micro-surfacetexturing apparatus and method to this end for efficiently andeconomically fabricating micro surface textures, such as micro channels,micro dimples or other depressions, on the surface of thin metallic ornon-metallic sheet material or internal surfaces of a journal bearing,bushing and the like.

In an illustrative embodiment, the present invention providesdeformation-based micro-texturing apparatus and method that embody upperand lower rolls, at least one of which has one or more micro surfaceprotrusions to plastically deform the surface of the workpiece (i.e.thin sheet material or journal bearing or bushing), by rolling action ina roll gap between the rolls, thereby creating arrays of micro surfacedepressions in the surface. The deformation method is controlled to formthe micro surface depressions without reducing the original thickness ofthe workpiece. The micro-deformation apparatus and method can involvepassing an electrical current through the workpiece to assist inmicro-deformation.

A deformation-based micro-texturing apparatus pursuant to an embodimentof the invention comprises a housing having first and second flexurebearing houses for rotatably supporting opposite ends of each of a first(e.g. upper) roll and a second (e.g. lower) roll to provide a workingroll gap between the rolls, wherein at least one of the rolls has one ormore micro surface protrusions to plastically deform a surface of aworkpiece by rolling action in the roll gap. A roll gap adjusting deviceis operably associated with the first and second flexure bearing housesof at least one of the rolls for adjusting the roll gap dimension withnanometer resolution to the selected final depth of the micro surfacedepressions to be imparted to the workpiece surface by the rollingaction.

In a particular illustrative embodiment, the deformation-basedmicro-texturing apparatus comprises a main mono-block (monolithic)housing having integral upper and lower flexure bearing houses tosupport each end of an upper roll and each end of a lower roll in amanner that enables at least one roll to move up and down. Each roll isrotated about its respective rotational axis by a respective motor,although one roll can act as a free wheel for certain applications. Oneor both of the upper and lower rolls has a micro surface texture to beimparted to the workpiece surface. When both rolls have micro surfacetextures, the surface textures can be the same or different. A coarseroll gap adjusting device is operably associated with the lower roll anda fine roll gap adjusting device is operably associated with the upperroll. For purposes of illustration and not limitation, the coarse rollgap adjusting device comprises a wedge adjusting device that movesadjacent flexure bearing houses of the lower roll in the verticaldirection to coarsely adjust the working gap between the two rolls,while the fine roll gap adjusting device comprises electric (e.g.piezoelectric) actuators, that, when energized, move the adjacentflexure bearing houses of the upper roll in the vertical direction toprovide fine adjustment of the roll gap dimension for initial set-up andalso during real-time surface texturing of the workpiece. A closed loopcontrol system includes a control unit for controlling the fine roll gapadjusting device to maintain a constant distance between the upper andlower flexure bearing houses, and thus provides a constant roll gapdimension between the rolls, in response to input signals fromnoncontact position sensing devices that detect the gaps between the twosets of upper and lower flexure bearing houses and feed correspondingsignals to the control unit for a closed-loop control of the fine rollgap adjusting device. A load detecting device is provided for detectingthe load applied to the flexure bearing houses by the fine adjustingdevice.

In practice of an illustrative method embodiment of the invention, ametallic or non-metallic (e.g. polymer) sheet material is surfacetextured by rolling action between the rolls with the roll gap adjustedto achieve a final depth of the micro surface depression(s) in thematerial surface with a small reduction in the original thickness of thesheet material. The sheet material can have an original sheet thicknessin the range of about 20 μm to about 1.5 mm, although the sheetthickness is dependent on the depth to which the sheet material is to betextured by depressions. The method can be practiced to form one or moremicro surface depressions, such as micro channels or micro dimples,having a lateral dimension (e.g. diameter or width) of about 1 μm toabout 10 mm and a depth of about 1 μm to about 10 mm. Typically, themicro depressions have a lateral dimension (e.g. diameter or width) ofabout 1 μm to about 100 μm and a depth of about 1 μm to about 100 μpm.

The present invention is advantageous in that the rolling method isdeformation-based such that there is a minimum material waste. Moreover,practice of the present invention enables mass production ofmicrochannels or other depressions on the workpiece surface at a lowcost and reduces the energy consumption significantly. The presentinvention is further advantageous in providing a micro-deforming methodthat can be practiced at room temperature without the need for heatingof the workpiece (e.g. sheet material or journal bearing) and issignificantly faster and less expensive than machining or lasertexturing. Compared to photochemical machining, micro-form rollingpursuant to the invention is more environmental friendly and moreefficient. The micro-form rolling technique could also be implemented inindustry to rapidly and inexpensively effect surface texturing of partsfor energy generation and energy utilization, greatly improving theviability of widespread use of surface texturing for friction reduction.

Other advantages of the present invention will become more readilyapparent from the following drawings taken with the following detaileddescription of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are respective schematic front and side elevation viewsof a desktop deformation-based micro-surface texturing apparatuspursuant to an embodiment of the invention. FIG. 1C is more detailedschematic side elevation view of the apparatus.

FIG. 2 is a schematic view of an exemplary flexure bearing house for aroll to allow a certain range of vertical motion of the roll as shown.

FIG. 3A is view of the fine roll gap adjusting device (shown aspiezoelectric actuators) for the upper flexure bearing house of theupper roll, while FIG. 3B is a view of the coarse roll gap adjustingdevice (shown as a wedge adjusting device) for the lower flexure bearinghouse of the lower roll.

FIG. 4 is a schematic partial perspective view of a noncontact positionsensor and its fixture for the flexure bearing houses.

FIG. 5A is a schematic end elevation and side elevation view of theupper roll having a protruding micro-channel-forming texture shown asprotruding ribs. FIG. 5B is a schematic end elevation and side elevationviews of the lower roll having a protruding surface texture featuresshown as rectangular protrusions.

FIG. 6A schematically illustrates a method embodiment of the inventionfor surface texturing a thin sheet material between the rolls and showsthe resulting surface textures on the upper and lower surfaces of thesheet material.

FIG. 6B schematically illustrates a method embodiment of the inventionfor surface texturing a journal bearing or bushing between the rolls andshows the resulting surface textures on the inner and outer surfaces ofthe journal bearing or bushing.

FIGS. 7A and 7B are WLI images of microsurface textures formed on AA5052aluminum sheet of Example 1.

FIGS. 8A, 8B, 8C, and 8D are WLI images of microsurface textures(microchannels) formed on various polymer (PP, Nylon, LDPE, and PMMA)sheets of Example 2.

FIG. 9 is a schematic view of a deformation-based micro-surfacetexturing apparatus pursuant to another embodiment of the inventionwherein an electrical power supply is connected to the sheet material toachieve electrical-assisted surface texturing pursuant to another methodembodiment of the invention.

FIG. 10 shows the profile of microchannels formed by theelectrical-assisted surface texturing (EAST) method.

FIG. 11 shows thickness of the sheet after texturing by the EAST method.

FIG. 12 shows stainless steel stress strain curves.

FIG. 13 shows the highest temperature reached during EAST.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a compact deformation-based micro-surfacetexturing apparatus as well as method for efficiently and economicallyfabricating surface textures, such as micro channels and micro dimples,on the surface of thin metallic or non-metallic sheet material or oninternal surfaces of a journal bearing, bushing or other antifrictionelement for providing the contact pair with a sufficiently low friction.The compact deformation-based micro surface texturing apparatus can beplaced on a desktop, work table, or other support surface and takes uplittle space compared to other equipment for micro texturing of sheetmaterials. The roll itself can be made with its micro surface texturethrough micro machining, electro discharge machining (EDM) or laserablation. The present invention enables mass production of micro surfacetextures at low cost and can reduce the energy consumption significantlyand is advantageous in that the micro-deforming method can be practicedat room temperature without the need for heating of the workpiece (e.g.the sheet material or journal bearing).

An illustrative embodiment of the present invention provides adeformation-based micro-texturing apparatus and method that embody upperand lower rolls, at least one of which has a micro-surface features, toplastically deform the surface of the workpiece by rolling action in aroll gap between the rolls, thereby creating arrays of surfacemicro-features (depressions) in the surface. The rolls are mounted onflexure bearing houses of a main housing and are actuated in a manner toprovide ultra-high precision and closed-loop controllability of the rollgap dimension to correspond to a selected final depth of the microsurface depression(s) to be formed in the surface of the workpiece bythe rolling action.

Referring to FIGS. 1A, 1B, and 1C, an illustrative apparatus embodimentof the present invention provides deformation-based micro-texturingapparatus having first (upper) and second (lower) rolls 10, 12, at leastone of which has a micro-surface protrusions P thereon, FIGS. 5A, 5B, toplastically deform the surface of the workpiece W, FIGS. 6A, 6B, byrolling action in the roll gap G between the rolls 10, 12, therebycreating arrays of surface micro depressions in the surface. Thedeformation method is controlled to form the micro-surface depressionswithout reducing the original thickness of the workpiece. Each roll isrotated about its respective rotational axis by a respective motor M1,M11. This system is capable of driving two rolls independently or justdrive one and leave the other one as a free wheel.

The deformation-based micro-texturing apparatus comprises a rigid mainhousing or frame 30 having integral upper and lower flexure bearinghouses FBH to support each end 10 a of an upper roll 10 and each end 12a of a lower roll 12 in a manner that enables at least one roll to moveup and down within a certain range. The main rigid housing or frame withintegral FBH's can be cut from one piece of high strength steel blockusing wire EDM to provide a rigid mono-block (monolithic) frame toensure ultra-high precision of the apparatus as described below,although the rigid housing or frame can be made by other methods such asby casting, forging, welding and the like.

The FBH's serve as support mechanism as well as guiding mechanism forthe upper and lower rolls 10, 12. To this end, each FBH has an opening34 with a conventional anti-friction bearing 35 to receive an end 10 aor 12 a of a respective roll 10 or 12 as shown in FIGS. 1B and 4.

FIG. 2 schematically illustrates a representative FBH, which is linkedto the main rigid housing or frame 30 with flexure hinges 32. The FBH'sprovide ultra precision vertical linear motion in a small range; forexample, a range of 1 μm to 1 mm. Flexure hinges 32, with low rotationalstiffness about one axis and high stiffness in other degrees of freedom,enable the FBH's to move vertically while keeping other degrees offreedom constrained. The FBH's possess high natural frequencies, vacuumcompatibility and clean room compatibility. Compared with a traditionalbearing house which is assembled with linear bushing and sliding rodsystem, the FBH's have no assembly, therefore, having no erroraccumulation, no backlash, no friction and no need for lubrication.

Stiffness plays a considerable role in the precise performance offlexure-based ultra-precision FBH's. Usually high stiffness is requiredto get ultra precise movement. Moving range is another design parameterto consider. The moving range is limited by the yield strength of theFBH material. The flexure hinges 32 will be damaged if the local stressof flexure hinge is higher than the yield strength of the material.Usually, once the material is selected, a compromise exists between thestiffness and moving range. The upper FBH's are pressed by actuators asdescribed below for a real-time roll gap control, so they need highstiffness to ensure the stability and high accuracy. The lower FBH's aresupported by the gap adjusting wedge block mechanism described below, sothe stiffness can be reduced to get larger moving range. The naturefrequencies of both the upper and lower FBHs are designed to be muchhigher than the frequency of texturing process itself in order to avoidthe resonance of the system. Finite element method (FEM) modeling is aneffective computational method to investigate the stiffness of a flexuremechanism.

One or both of the upper and lower rolls 10, 12 has a micro surfaceprotruding texture to plastically deform the surface of the workpiece W,creating an array of micro surface depressions with a rolling action.Each roll 10, 12 can be made with the desired surface texture throughmicro machining, electro discharge machining (EDM) or laser ablation.Roll with surface features may be made of hard metals, tungsten carbide,aluminum, ceramics of nitrogen silicon and the like. FIG. 5A shows roll10 or 12 which is formed with micro channel-forming protruding features11. FIG. 5B shows roll 10 or 11 which is formed with protrudingcorrugation features P, which comprises alternately arranged recessesand projections. Preferably, each micro channel-forming protrusion orrecess has a depth of 5 to 100 μm and a width of 2 to 100 μm. Inpractice of the invention, one roll can have a suitable protruding microsurface texture, while the other roll is polished smooth. Alternately,both rolls 10, 12 can have a micro surface textures, which can be thesame or different, such that both sides of a workpiece are textured.

The apparatus includes at least one roll gap adjusting device that isoperably associated with the FBH's of at least one of the rolls 10, 12for adjusting the roll gap vertical dimension D with nanometerresolution corresponding to the selected final depth of the microsurface depressions to be imparted to the surface of the workpiece bythe rolling action. For purposes of illustration and not limitation andreferring to FIGS. 1C and 3B, a coarse roll gap adjusting device 100 isprovided and comprises a wedge adjusting unit having wedges 110 a andadjusting screw 112 that moves the flexure bearing houses FBH's of thelower roll 12 in the vertical direction to coarsely adjust the workingroll gap dimension between the two rolls, while a fine roll gapadjusting device 120 comprises electric (e.g. piezoelectric) actuators122, that, when energized, move the flexure bearing houses FBH's of thatupper roll 10 in the vertical direction to provide fine adjustment ofthe roll gap dimension D between the rolls for initial set-up and alsoduring real-time surface texturing of the workpiece.

The roll gap G between upper and lower rolls is first adjusted using thelower coarse roll gap adjusting device 100 by pushing forward or pullingback the bottom wedge 110 using screw 112 of the two wedges shown.Afterward, the upper fine roll gap adjusting device 120 with nanometeraccuracy is used for fine adjusting and for real-time adjusting duringthe surface texturing process to correspond to the selected final depthof the micro surface depressions to be imparted to the surface of theworkpiece by the rolling action.

As is seen from FIGS. 1B, 1C and 3A, the fine roll gap adjusting device120 comprises, for each FBH of the upper roll, a piezoelectric actuator122 mounted on the frame 30, a piezoelectric load cell 123, and a balltip 124. A pair of actuator/load cell/ball tip stacks thus are employedto press the two upper flexible bearing houses FBH's of the upper rollindependently for ultra precise close-loop control of the gap betweenthe upper and lower rolls and to prevent the tilting of the roll. Eachpiezoelectric load cell 123 is installed between the piezoelectricactuator 122 and the ball tip 124 to monitor the real-time rollingforce. Compared to common strain gauge load cells, the loadpiezoelectric cells 123 have super high stiffness, which is important tothe keep the ultra high precision of the whole apparatus. They arepre-loaded to approximately 20% of the measuring range in order toobtain the best possible linearity. The ball tips 124, which are fixedin the punch head of the piezoelectric actuators 122, are used toprevent the tilting and shearing force which can damage thepiezoelectric actuator.

The roll gap G between two rolls 10, 12 cannot be measured directly.Instead, two noncontact capacitive position sensors 130 (one shown) areused to measure the gaps between upper and lower flexure bearing housesFBH's at both ends of the rolls 10, 12 separately. In this way, the rollgap G between the two rolls 10, 12 can be calculated and any minortilting of the roll can be detected very easily. FIG. 4 shows eachnoncontact position sensor 130 affixed to the rigid housing or frame 30so that each sensor 130 is associated with the gap between upper andlower FBH's of the rolls 10, 12. A sensor fixture F is used to fix thesensor and to adjust the initial gap between the sensor the target.Signals from each position sensor 130 are fed into a control unit C(e.g. microprocessor), so that the stroke of piezoelectric actuators 122is carried out based on (in response to) the signal to keep the roll gapG between upper and lower rolls a required constant value.

A closed loop control system for the piezoelectric actuators 122includes the control unit CONTROLLER for controlling the fine roll gapadjusting device 120 to maintain a constant distance between the upperand lower flexure bearing houses FBH's and thus provides a constant rollgap dimension between the rolls 10, 12 in response to input signals fromthe noncontact position sensing devices 130.

In the following description, with the aid of FIG. 6A, operation ofmicro surface texturing apparatus will be described. In the first case,sheet material W is properly sandwiched between the upper and lowerrolls 10, 12. Before this, by work of the roll gap adjusting device, thegap G between upper and lower rolls 10, 12 is adjusted according to therequired final depth of the micro surface features. The piezoelectricactuators 122 are utilized to get nanometer accuracy of the roll gapdimension D. Signals from the position sensors 130 are read by thecontrol unit CONTROLLER to monitor the gap G and maintain the gap atdesired constant value. CONTROLLER receives load signals from the loadcells 123. With continuous rolling process effected by counter rotationof rolls 10, 12 and passing of the sheet workpiece through the roll gap,the micro surface features of the textured roll are pressed against thesurface of the sheet workpiece with a certain pressing force and aretransferred to the sheet surface.

In the second case shown in FIG. 6B, a journal bearing or bushing W istextured on the inner surface. The upper roll 10 is inside the bearing,pressing the bearing against the lower roll 12 which is outside thebearing. In this case, the upper roll 10 is textured while the lowerroll is not and instead is smooth. The micro surface features of theupper roll 10 are pressed against and are transferred to the innersurface of the bearing or bushing by rolling action of rolls 10, 12.

EXAMPLE 1

To validate the deformation-based surface texturing method of theinvention, experiments were conducted using the apparatus describedabove with the main rigid housing or frame 30 with integral FBH's cutfrom one piece of high strength steel block using wire EDM to provide arigid mono-block (monolithic) frame. The rigid frame 30 had an outerenvelope having compact dimensions of 340 mm, 200 mm and 166 mm suchthat it could be placed on a desktop.

One surface of different AA5052 aluminum sheets (40 mm width and 100 mmlength) were textured with micro channels to different depths. The rollgap between two rolls 10, 12 was adjusted to correspond to each requiredfinal depth of the micro channels. The rolls 10, 12 were 40 mm in lengthand 46 mm in diameter and were made of tungsten carbide. Piezoelectricactuators 122 were utilized to press the FBH's of the upper roll 10 toget nanometer accuracy of the roll gap dimension D. Signals fromdisplacement sensors 130 were read by the control unit to monitor theroll gap simultaneously. Then, by the continuous rolling process usingcounter rotating rolls 10, 12, the upper roll 10 is pressed toward thesheet. With this, the micro channel features of the roll 10 are pressedagainst the contact surface of the sheet with a certain pressing forceand are transferred to the sheet. Two electric motors counter rotate theupper and lower rolls independently, giving them speeds up to 20 rpm.The upper roll had a protruding texture of 100 μm wide and 100 μm deepmicro channel-forming ribs; the lower roll was polished without anysurface features. Two piezoelectric actuators 122 were capable ofproviding up to 60 kN force and 150 μm stroke length with 0.8 nmresolution. The stiffness of the piezo actuators is 360 N/μm and theresonant frequency is 4.5 kHZ. The piezoelectric load cell 123 had anultra high rigidity of 5 kN/μm and was capable of measuring up to 30 kNload. The capacitive position sensors 130, with a measurement range upto 2 mm and a resolution as small as 10 nm, were used for non-contactposition measurement.

A Zygo white light interferometer (WLI), which can scan a depth of 150μm with a minimum resolvable depth less than 1 nm; was used to examinethe features (depressions) on the formed sheets. FIGS. 7A, 7B show theWLI images of the textured surfaces. FIG. 7A shows the micro channelswith depth as 25 μm and width as 100 μm; FIG. 7B shows the cross-channelpattern made by texturing the sheet at perpendicular directions (rollingthe sheet first in one direction and then in a perpendicular direction).

EXAMPLE 2

Other experiments were conducted using the apparatus of Example 1 totexture soft polymers including PP, Nylon, LDPE, and PMMA in sheet formhaving a thickness of 1 mm.

FIGS. 8A, 8B, 8C, and 8D are WLI images of the microsurface textures(microchannels) successfully formed on the various PP, Nylon, LDPE, andPMMA polymer sheets of Example 2. The mcirochannels had a widthdimension of 100 μm and depth dimension of 25 μm.

Electrical-Assisted Micro Surface Texturing

The present invention further envisions another embodiment referred toas electrical-assisted surface texturing (designated EAST) useful, ifneeded, to texture certain hard metals, such as for example stainlesssteel and titanium, having high strength and large elastic recovery.EAST involves passing a continuous or pulsed electrical current (DC orAC) through the workpiece during micro deformation. Electrical-assistedforming can significantly improve the formability of the hard metal andreduce springback due to the electroplastic effect. Pulsed current hasthe advantage of reaching higher current density while keeping thetemperature of the workpiece relatively low. However, the formed surfacetextures will have different depth when the current is on and off. Tomeet the requirement of uniform texture depth, continuous electricalcurrent is used in EAST with proper insulations and ground connections.Since the materials to be textured are conductive metals, the requiredvoltage is minimal.

In practicing the EAST embodiment of the invention, the rollingapparatus described above having monolithic frame 30 with FBH'ssupporting the upper and lower rolls 10, 12 was modified to provideelectrical current that travels from the electrical contact 200, or theanode, to the sheet workpiece W to the textured upper roll 10 (or thecathode) as illustrated in FIG. 9.

The effect of electricity on formability of micro surface texturingprocess will be described in the Example 3 below. Increasing theformability has several potential benefits, such as allowing morecomplex and deeper textures to be fabricated, reducing the springbackand pile up and reducing the rolling force. In this Example 3, the majorelectrical current passes through upper roll, as shown in FIG. 9. Basedon this setup, experiments were conducted to evaluate the effect ofelectrical current.

EXAMPLE 3

Stainless steel strips with a width of 5 mm and thickness of 1.2 mm wereused in the experiments to study the ESFT process. The rolling speed was2 mm/s. Three cases of current densities through the contact area of theteeth of the upper roll and the sheet (strip) were studied, which were15 A/mm², 20 A/mm² and 25 A/mm². The required currents passed throughthe sheet were 90 A, 120 A and 150 A, respectively. With electricalcurrent of 0 A, 90 A, 120 A and 150 A, the final depth of the microchannels are 34.5 μm, 38.5 μm, 44.6 μm, and 58 μm, respectively, FIG.10. With 150 A current pass through, the depth of micro channelincreased more than 68%.

Besides of increasing the depth of textured micro channels, thethickness of the sheet also reduced with electrical effect. For eachcase, five samples were textured, and the final thickness of the samplewas measured. Without electrical current, the average final thickness ofthe sheet is 1.15 mm; while, with electrical current 90 A, 120 A and 150A, the average final thicknesses of the sheets are 1.14 mm, 1.13 mm and1.11 mm, respectively, FIG. 11. This result is due to the electricaleffect on reduction of yield stress of the stainless steel.

The experimental results matched the strain-stress curve reported byPerkins et. al, “Metallic forging using electrical flow as analternative to warm/hot working”, Transactions of the ASME, vol. 129,2007, which is shown in FIG. 12 The effect of the electrical current onstainless steel was significant at relatively low current densitiescompared to the other materials examined. Even at a density of 17.8A/mm², the workability of the specimen significantly increased. Fromboth FIGS. 10 and 12, it is clear to see that when the current densityis higher than a certain value, a small change of current density willlead to a more significant change of stress. To be noted, in theirelectrical-assisted compression test, the highest temperature reachedwas 182° C. In the next section, the temperature of the sample will bediscussed.

Temperature Measurement:

Passing electrical current through the sample increases the temperature.The temperature of the sample during EAST was measured and proved to betoo low to lead to such a significant increase of the channel depth.Infrared camera was used to monitor the temperature during EAST. As oneexample, the temperature contour during the EAST with 120 A electricalcurrent pass through the sheet was observed. At the beginning oftexturing, the temperature of the sheet was 27.4° C. which is close tothe room temperature. During the texturing, with electrical current passthrough the stainless steel sheet, the highest temperature reached up to136.5° C., FIG. 13, which is still far below the melting temperature ofthe stainless steel, 1510° C.

The present invention provides micro surface textures having variousapplications, such as friction/wear reduction, bacteria sterilization,and self-cleaned surfaces. The present invention, a desktop microsurface texturing apparatus, can efficiently and economically fabricatesurface textures such as micro channels and micro dimples on thesurfaces of thin sheet materials or internal surfaces of a journalbearing. The present invention will have applications in the fields oftribology, for contact surfaces such as journal bearings, and otherinterfaces such as turbine blades in jet engines, and part and toolinginterface in any moving pairs. Thin sheet with micro channels can alsobe stacked to make arrayed microchannel device (AMD) that facilitatesheat exchange or mass transfer. AMD has a direct impact on increasingenergy efficiency in the application areas, such as distributed powergeneration, hydrogen generation and fuel cells

Although the invention has been described with respect to certainembodiments, those skilled in the art will appreciate that modificationsand changes can be made therein within the scope of the invention asdefined by the appended claims.

REFERENCES

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1. A deformation-based micro-texturing apparatus, comprising a housinghaving first and second flexure bearing houses for rotatably supportingopposite ends of each of a first roll and a second roll to provide aroll gap between the first roll and the second roll, wherein at leastone of the first roll and the second roll has one or more micro surfacefeatures to plastically deform a surface of a workpiece deformed byrolling action in the roll gap, and a roll gap adjusting device operablyassociated with the first and second flexure bearing houses that supportthe opposite ends of the at least one of the first roll and the secondroll for adjusting the roll gap to the final depth of the micro surfacefeatures to be imparted to the surface of the workpiece by said rollingaction.
 2. The apparatus of claim 1 wherein the first roll is an upperroll and the second roll is a lower roll, one or both driven to rotateby a respective motor.
 3. The apparatus of claim 2 wherein the roll gapadjusting device comprises first and second electric actuators thatpress on the first and second flexure bearing houses associated with theopposite ends of the at least one of the first roll and the second rolland that, when energized, move the adjacent first and second flexurebearing houses in a vertical direction to adjust the roll gap forinitial set-up and during real-time surface texturing of the workpiece.4. The apparatus of claim 3 wherein the roll gap adjusting devicefurther comprises a closed loop control system that includes a controlunit for controlling the roll gap adjusting device to maintain aconstant distance between the upper and lower flexure bearing houses,and thus a constant roll gap dimension in response to input signals fromnoncontact position sensing devices associated with gaps between saidupper and lower flexure bearing houses.
 5. The apparatus of claim 4wherein the roll gap adjusting device further comprises a load detectingdevice associated with each electric actuator for detecting the loadapplied to the adjacent flexure bearing house.
 6. The apparatus of claim4 wherein the first and second electric actuators comprisespiezoelectric actuators.
 7. The apparatus of claim 3 wherein the rollgap adjusting device further includes a wedge coarse roll gap adjustingdevice operably associated with the other of the first roll and secondroll.
 8. The apparatus of claim 1 wherein both the first roll and secondroll have one or more micro features to plastically deform the surfaceof a workpiece deformed by rolling action in the roll gap,
 9. Theapparatus of claim 1 wherein the housing is a monolithic frame integralwith the first and second flexure bearing houses associated with each ofthe first roll and the second roll.
 10. The apparatus of claim 1 furtherincluding a power source connected to the workpiece for passing anelectrical current therethrough during deformation.
 11. A method forsurface texturing a metallic or non-metallic material, comprisingpassing the material through a roll gap formed between opposing rolls,at least one of which has one or more micro surface protrusions, andforming one or more micro surface depressions in the surface by saidmicro surface protrusion plastically deforming the surface by rollingaction.
 12. The method of claim 11 wherein the material is a sheetmaterial.
 13. The method of claim 12 wherein the sheet material has asheet thickness in the range of about 20 μm to about 1.5 mm.
 14. Themethod of claim 11 wherein the material is an antifriction element. 15.The method of claim 11 wherein the micro surface depressions have alateral dimension of about 1 μm to about 10 mm and a depth of about 1 μmto about 10 mm.
 16. The method of claim 15 wherein the lateral dimensionis about 1 μm to about 100 μm and the depth is about 1 μm to about 100μm.
 17. The method of claim 11 further including pressing a pair offlexure bearing houses supporting opposite ends of one of the rolls in adirection to adjust the roll gap dimension to achieve the final depth ofmicro surface depressions in the material surface.
 18. The method ofclaim 17 further including sensing gaps between adjacent sets of flexurebearing houses supporting opposite ends of the rolls and adjusting theroll gap dimension during surface texturing in response to the sensedgaps.
 19. The method of claim 11 further including passing an electricalcurrent through the workpiece as the material passes through the rollgap.
 20. A sheet material having at least one surface that includesrolled-in micro surface depressions.
 21. The sheet material of claim 20wherein the micro surface depressions have a lateral dimension of about1 μm to about 10 mm and a depth of about 1 μm to about 10 mm.
 22. Thesheet material of claim 21 wherein the lateral dimension is about 1 μmto about 100 μm and the depth is about 1 μm to about 100 μm.
 23. Anantifriction element having an inner surface that includes rolled-inmicro surface depressions.
 24. The element of claim 23 wherein the microsurface depressions have a lateral dimension of about 1 μm to about 10mm and a depth of about 1 μm to about 10 mm.
 25. The element of claim 24wherein the lateral dimension is about 1 μm to about 100 μm and thedepth is about 1 μm to about 100 μm.